(a) Field of the Invention
The present invention relates to a method of making micro-holes on a metal plate, in particular to a method of making a maximum of micro-holes per unit area on a metal plate.
(b) Description of the Related Art
In the present living environment, various different noises are produced, which affect the quality of our living significantly, so that all kinds of sound absorbing or isolating devices are introduced to solve the noise problem. Among these devices, a sound gobo has an excellent sound absorbing effect, and the structure of the sound gobo is originated from the famous Chinese academician, Mr. Ta-yu Ma's “Micro perforated sound absorption panel theory” in 1970, and the theory primarily forms a plurality of micro-holes on a surface of a panel, wherein the diameter of the micro-hole is smaller than the thickness of the panel, such that after a sound enters into the micro-holes (tunnels), kinetic energy of sound wave and air molecules will pass through the center of the tunnels quickly and attach onto the walls of the tunnels. Friction produced by the molecules will attenuate the sound until the kinetic energy of the molecules is converted into heat energy, so as to achieve the sound absorption effect. The inventor of the present invention based on this theory has obtained an issued patent (Taiwan Utility Model Pat. No. M289784, entitled “Metal sound gobo” on Apr. 21, 2006, and the metal sound gobo of the patented invention comprises a plurality of triangular cones, having an elliptical micro-hole at the bottom of each triangular cone and concavely formed at the bottom of a metal plate, a slightly wave-like surface formed at the top of the metal plate, and a triangular cone concavely formed around the periphery at the top of the wave-like surface and disposed at a position corresponding to the elliptical micro-hole, such that the reflected sound waves are attenuated by their collision and interference with each other. In the meantime, even if some of the sound waves pass through the elliptical micro-holes formed at the bottom of the triangular cones, an acoustic transmission loss will occur to achieve a better sound absorption and a quicker assembling effect.
The inventor of the present invention has further filed a patent application (Taiwan Patent Application No. 200920902, entitled “Geometric micro-hole sound gobo” on May 16, 2009, and the geometric micro-hole sound gobo of the patent application comprises a metal plate installed at the bottom of a floor layer, and a micro-hole camber and a geometrical micro-hole groove concavely and respectively formed on the top and bottom of the plate and interconnected with each other, such that refractions occurred at conical surfaces of different angles promotes the interference phenomenon and depletes the kinetic energy of air molecules, and an air layer between the plate and the floor layer can increase the friction loss of the kinetic energy of the sound waves, so as to achieve a good sound absorption effect.
However, both of the aforementioned patent and patent application use the “micro-hole panel sound absorption theory” and common sound gobo available in the market also comes with the structure manufactured and produced according to this theory. Since the sound-absorption rate is related to the quantity of micro-holes per unit area of the panel (or plate), therefore a maximum of micro-holes formed on the plate not only improves the sound-absorption rate, but also saves material and manufacturing costs.
Most of conventional sound gobos adopts the manufacturing technique of using a punching machine to punch holes on a plate directly. The direct punching process can produce 40000 to 50000 micro holes per every square meter on the plate, but the minimum diameter of each micro hole can reach 0.45 mm only, and thus it is difficult to punch more holes with a smaller diameter on unit area of the sound gobo. As a result, the average noise reduction coefficient (NRC) can reach 0.15 to 0.5 (wherein, the less the numerical value of NRC, the better is the sound-absorption rate).